In recent years, in-vehicle image processing devices capable of determining whether there is a parkable area on the basis of an image of a vehicle-mounted camera have been developed. As an example of this kind of device, there is known a device for estimating width of a vehicle (refer to PTL 1, for example).
The device described in PTL 1 estimates width of a vehicle according to a procedure below to determine whether there is a parkable area. First, an image of the vehicle is taken, and a wheel and a license plate of the vehicle are recognized from the image taken. Next, a first point in the recognized license plate is projected on a virtual plane parallel to a road surface. Subsequently, second and third points on the recognized wheel along the vehicle longitudinal direction are projected on the virtual plane and a first line is set so as to connect the second and third points to each other. Next, a second line extending from the first point is set so as to be orthogonal to the first line and an intersection of the first line and the second line is calculated. Then, width of the vehicle is estimated on the basis of a distance between the intersection and the center of the recognized license plate in a vehicle lateral direction.